WILLIAMSPORT, Pa. -- Halliburton is building a permanent outpost here on the edge of a one of the 21st century's biggest energy booms.

...Halliburton is a ubiquitous presence in the world's biggest oil fields. For the past two months, it has defended itself against charges that shoddy cement work contributed to a methane blast that sank BP's rig in the Gulf of Mexico and killed 11 people. As long as the well keeps gushing, public anger could weaken America's appetite for offshore drilling.

But far from the Gulf Coast and outside of the media spotlight, Halliburton and the oil and gas industry are spending billions of dollars in preparation for decades of drilling in the Marcellus Shale. The 95,000-square-mile sheet of natural gas-rich sediment sprawls across Pennsylvania, southern New York, West Virginia and eastern Ohio.

Pittsburgh-based Atlas Energy Inc. in April formed a $1.7 billion partnership with Reliance Industries Ltd., the largest private-sector company in India. The conglomerate is controlled by Indian billionaire Mukesh Ambani, who has been pushing the company to secure lucrative energy investments outside of India.

"In the last few years, we realized we had this extremely valuable asset," says Jeff Kupfer, senior vice president of Atlas. "We needed a lot of capital to develop it." Once Atlas put out a feeler, the Marcellus prospect attracted attention from the world's major oil and gas companies. "There was something in the chemistry with Atlas and Reliance."

The $31 billion Exxon-XTO all-stock deal still has to jump some regulatory hurdles. If the merger becomes real, Exxon will be the largest natural gas producer in the country, controlling large chunks of acreage in the most promising onshore gas fields in the United States.

Texas, Louisiana, Oklahoma, Arkansas and the Appalachian regions of Pennsylvania and New York are the epicenter of shale gas, coalbed methane and tight-sand gas formations.